In the present work we will try to analyze the existing relation between the banking concentration, the information asymmetries in the financial markets, and the credit rationing to the sector of the small and medium companies (SMEs). As departure point we’ll based on the several works on asymmetries of information and rationing of credit published since the already classic work of Stiglitz and Weiss (1981), and we’ll take as empirical frame the situation of the financial and SME sectors in Argentina after the called `Tequila Crisis’ (1994) up to year 2001, since we think that this period holds the necessary conditions to fit the three subjects.

It is well known that the financial markets and the banks in particular are informational intensive activities and depends critically on their quality and amount, reason why the information deficiencies have a superlative incidence in their correct operation. But this correct operation gains even more importance if we consider that this sector has a vital function as financial intermediary, connecting the savings of the public with the demand for investment, fulfilling a fundamental role in the growth and development of any economy. It is normally accepted that the greater it is the amount and quality of the information the grater will be the resources canalized towards those opportunities of investment, nevertheless the evidence of the financial markets not always fulfils the golden rule of equilibrium between the supply and demand.

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It is then when the aspects and situations that we’ll mention in the present work starts to gain relevance. The information deficiencies caused by endogenous factors like the particular structure of the small company, or exogenous factors like the banking concentration and the impact of the regulations (as we’re going to see further), leads to phenomena known as `adverse selection', `moral risk' or `monitoring costs', causing `credit rationing' on some sectors of the market.

In the first part of this work we will review the basic characteristics of the process that causes the credit rationing, highlighting the information asymmetries. We’ll relate them later with the banking concentration that took place and the effects of the financial regulations applied over the mentioned period.

We will continue getting deeper into the analysis, focusing on the difficulties that the financial institutions had to solve the problem of `information asymmetries' and will see the evidences of credit rationing over the most affected sector that was the small and medium companies one. For this task we will consider the size and structure of the financial organizations, type of information used and the relation with the SME sector with their singularities.

In the third part we will try to analyze the microeconomic impact that the credit rationing had in the small and medium companies. We will lean on empirical evidence and perform a simulation that allows us to quantify the financial over-cost that the rationing provokes in a particular company.

In summary, the intention is to relate the general theory of credit rationing' to the real effect on minor companies, analyzing the agents which somehow fulfil a role of intermediaries.

Finally it is important to clarify that the present work does not try to demonstrate the viability of lending to a SME but the costs and obstacles to do it.